A. Field of the Invention
The invention generally relates to mask construction techniques employed in photographic fabrication of microminiature electronic devices, particularly those devices requiring the use of high resolution photolithographic techniques.
B. Description of the Prior Art
Pattern generation in planar thin film overlays is usually accomplished by reproducing the pattern (or its negative) in a photosensitive resist applied to the film's surface. In order to obtain accurate delineation of the pattern, positive contact between the pattern mask and the photoresist surface is essential. Build-ups of photoresist material often occur around the perimeter of a substrate and thereby prevent contact between the pattern mask and the surface of the photoresist. Thus, the precise masking ability of the mask apparatus is of great importance. In the prior art, methods for making precise mask apparatus range from chemical buildup of the mask to mechanical construction thereof. Chemically fabricated masks require a number of processing steps both in the application of the mask and in the removal thereof. Mechanical masks therefore present certain advantages, such as reduction of fabrication time, reusability, etc. However, mechanical masks in high resolution work are susceptible to the aforementioned difficulties associated with excessive photoresist deposits around the perimeter of a substrate which prevent intimate contact between the masking pattern and the surface of the substrate.
A relief mask, such as is disclosed in U.S. Pat. No. 3,824,014 can be used to provide the intimate contact required to produce accurate pattern generation. The present invention provides a method which is useful inter alia to fabricate such a relief mask. The method of the present invention is an improvement over the method for making a relief mask disclosed in U.S. patent application Ser. No. 473,107, filed May 24, 1974, now abandoned. In the method of said patent application Ser. No. 473,107, a thin layer of gold is deposited on a glass substrate after the usual cleaning of the substrate to enable photoresist material to adhere to the thin gold layer, which gold layer adheres to the glass substrate. As taught by the prior method, the gold layer was necessary to enable the adherence of the photoresist material to the substrate, the photoresist material not being capable of direct adhesion to the glass substrate. On development of the photoresist and removal of desired portions thereof, portions of the gold layer had to be removed to allow etching of selected areas of the surface of the glass substrate. Thus, several unit operations, i.e., the deposition of gold, the subsequent removal of portions thereof, and finally the removal of the remaining portions thereof, were required in the practice of the prior method which are no longer required due to the teachings of the present invention.